The present invention relates to a polyester film as a base film for sublimation-type thermosensitive transfer recording medium.
Sublimation-type thermosensitive transfer is typified by excellent gradation quality of the printed image in comparison with melt-type thermosensitive transfer, and is preferably employed for full color printing where distinct expression of intermediate colors is particularly required. Recently, through improvements of the recording media containing sublimable dyes or improvements of hardware devices such as printers, it has become possible to obtain images with high fineness almost equal to photograph.
Another serious request in full color printing using sublimation-type thermosensitive transfer is enlargement of the size of the printed image. Particularly, the printed images having a large area such as more than A-4 size are demanded. It is also required that the printing be performed in a time not much longer than needed for conventional A-6 to B-5 size printing. This calls for elevation of the printing speed and entails an increase of the printing energy.
However, in the conventional thermosensitive transfer recording media, the heat resistance of the base film has been unsatisfactory, and an increase of the printing energy would give rise to the problem of wrinkling of the ink ribbon under high heat, causing a printing defect (white line) at this portion to badly reduce the quality of the printed image.
In the course of the present inventors' studies for overcoming the above problem, it has been found that by using a polyester film having the specific properties as base film of the recording medium, more specifically by using, as base film of a sublimation-type thermosensitive transfer recording medium, a laminated polyester film obtained by applying a coating solution comprising at least one water-soluble or water-dispersible resin selected from the group consisting of urethane-based resins, polyester-based resins and acrylic-based resins on at least one side of a polyester film as a support film before the oriented crystallization of the polyester film is completed, and subjecting the obtained film to drying treatment, stretching treatment and heat-setting treatment. The present invention has been attained on the basis of the said finding.